In the fabrication of semiconductor devices, semiconductor materials may be used that are lattice mismatched with a substrate or underlying layer. For example, GaN is conventionally fabricated on a sapphire substrate, a silicon substrate or a silicon carbide substrate. The unstrained lattice constant of GaN is 3.19 whereas the unstrained lattice constant of sapphire is 4.76 and silicon carbide is 3.07. As a result, the GaN layers grown on the substrate may be strained. If such is the case and if the level of strain exceeds a certain threshold, the GaN layers may crack, which may render the material unacceptable for use in a semiconductor device.
Further difficulties may arise from the fact that different materials may have different coefficients of thermal expansion, which may cause the lattice constant differential between materials to change with temperature. Thus, two materials that are substantially lattice matched at one temperature may be mismatched at a different temperature. Furthermore, an epitaxial layer that is compressively strained at a growth temperature due to a lattice mismatch with an underlying substrate may be tensile strained at room temperature, or vice-versa, depending on the room temperature lattice constants and the coefficients of thermal expansion of the materials.
Process temperatures used in the fabrication of semiconductor materials and devices may be extreme. For example, some epitaxial growth processes may be performed at temperatures in excess of 1000° C., while device annealing temperatures may be even higher. Thus, the difference between process temperatures and room temperature may be as much as 1000° C. or more.
In addition to potentially causing cracking, strain in a semiconductor structure may be detrimental for a number of other reasons. For example, strain may cause a wafer to bow. Wafer bow during epitaxial growth may result in uneven growth of epitaxial layers on a substrate, which may reduce useful device yields. Furthermore, wafer bow may complicate semiconductor manufacturing processes such as planarization and/or dicing.